1. Field of the Invention
The present invention relates to an optical transmission line for transmitting light signals by using an optical fiber.
2. Related Background Art
WDM (Wavelength Division Multiplexing) transmission using an optical fiber network allows large-capacity, high-speed information transmission using WDM signals containing a plurality of wavelength components. As a transmission medium applied to this WDM transmission, an optical fiber having an effective area large enough to suppress the occurrence of a nonlinear phenomenon is preferable. An optical fiber serving as such a transmission medium is designed to have a small dispersion in terms of absolute value and a small dispersion slope in a 1.5-xcexcm band, which is the wavelength range of the WDM signals, so as to reduce the accumulated wavelength dispersion (to be referred to as dispersion hereinafter) of the WDM signals (see, for example, Japanese Patent Laid Open Nos. 8-234036 and 8-304655). In addition, an optical fiber susceptible to the adverse influence of bending easily causes the increase of transmission loss by manufacturing cable. For this reason, the optical fiber preferably has a low bending loss.
A nonlinear phenomenon means as a phenomenon in which a signal light pulse is distorted in proportion to the density of light intensity or the like owing to an optical phenomenon such as FWM (four-wave mixing), SPM (self-phase modulation), or XPM (cross-phase modulation). This phenomenon becomes a factor that imposes a limitation on transmission speed or repeater intervals in a repeater transmission system. Dispersion means as a phenomenon in which a pulse wave broadens because the propagation speed of light varies with wavelength. A dispersion slope is defined as the slope of a curve representing dispersion characteristics in a predetermined wavelength range.
The present inventors have found the following problem upon studying an optical fiber applied to a conventional optical transmission line.
In general, in an optical fiber designed to have a large effective area and a small dispersion in terms of absolute value, the bending loss is high. In order to decrease the bending loss, the cutoff wavelength must be increased. A cutoff wavelength longer than those of the light signals can be set by using the distance dependence of cutoff wavelength (e.g., T. Kato, et al., OECC""96, 17C3-4). In this case, however, mode coupling from a fundamental mode to high-order modes occurs at the node or the like between optical fibers, and a deterioration in transmission characteristics is caused by mode-dispersion. This also imposes a limitation on an increase in cutoff wavelength.
It has been difficult to design an optical fiber that can satisfy all the requirements, i.e., a large effective area, a smaller dispersion in terms of absolute value, and a low bending loss. In other words, an optical transmission line has not been realized yet, which can suppress the occurrence of a nonlinear phenomenon and dispersion with respect to all wavelength components contained in WDM signals and is resistant to bending (is not easily affected by the overall transmission characteristics).
The present invention has been made to solve the above problem, and has as its object to provide an optical transmission line having a structure which has a small dispersion and excellent transmission characteristics in optical transmission in a 1.5-xcexcm band and is suitable to WDM transmission.
An optical transmission line according to the present invention is provided at least between a transmitter and a receiver, between a transmitter and a repeater, between repeaters, or between a repeater and a receiver, and functions to transmit light signals containing one or more wavelength components. The optical transmission line according to the present invention comprises at least one transmission unit.
This transmission unit comprises: a dispersion-shifted optical fiber having an incident terminal on which light signals are incident, an exit terminal from which the light signals are emitted, and a cutoff wavelength longer than those of the light signals at a fiber length of 2 m, and a mode removing unit for reducing the optical power of high-order modes excluding a fundamental mode by an amount larger than transmission loss of high-order modes when the light signals propagate from a first portion to a second portion of the dispersion-shifted optical fiber.
Note that the first portion of the dispersion-shifted optical fiber corresponds to the incident portion of the mode removing unit, and the second portion of the dispersion-shifted optical fiber corresponds to the exit portion of the mode removing unit. The mode removing unit functions to satisfy the single-mode condition in optical transmission without using the distance dependence of cutoff wavelength in order to effectively reduce high-order modes.
The above dispersion-shifted optical fiber may be obtained by optically connecting a plurality of dispersion-shifted optical fibers to each other. In this case, the optical transmission line according to the present invention includes a plurality of transmission units. Each transmission unit comprises a dispersion-shifted optical fiber as a transmission medium and a mode removing unit associated therewith. Note that the mode removing unit prepared for each transmission unit may include a plurality of mode removing devices for removing high-order modes. These mode removing devices function as a whole to remove high-order modes propagating through the associated dispersion-shifted optical fibers.
In the optical transmission line according to the present invention, the above mode removing unit is arranged closer to the incident terminal side than the center of the dispersion-shifted optical fiber in the longitudinal direction of the dispersion-shifted optical fiber, and more preferably arranged near the incident terminal of the dispersion-shifted optical fiber. The exit terminal of another optical component, e.g., a dispersion-shifted optical fiber, optical fiber for transmission, or transceiver, is optically connected to the incident terminal of the above dispersion-shifted optical fiber. A position near the incident terminal therefore means a position immediately after the node between the exit terminal of another optical component and the incident terminal of the dispersion-shifted optical fiber. The dispersion-shifted optical fiber applied to the optical transmission line, in particular, has a cutoff wavelength longer than those of the light signals to reduce the bending loss. In order to ensure the single-mode condition, therefore, the mode removing unit is preferably positioned near the incident terminal of the dispersion-shifted optical fiber. When a plurality of dispersion-shifted optical fibers are optically connected to each other, mode-dispersion is likely to occur because optical power is exchanged between the fundamental mode and high-order modes at each node. In this case, coupling from high-order modes to the fundamental mode (the occurrence of mode-dispersion) can be effectively suppressed by removing high-order modes immediately after the node, and the propagation of high-order modes can also be suppressed. Note that coupling from the fundamental mode to high-order modes indicates that the transmission loss apparently increases.
The dispersion-shifted optical fiber applied to the optical transmission line according to the present invention has a cutoff wavelength longer than those of the light signals to reduce the bending loss. For this reason, in consideration of the distance dependence of cutoff wavelength as well, the single-mode condition is not satisfied near the incident terminal of the dispersion-shifted optical fiber. Reducing high-order modes is indispensable to satisfying the single-mode condition. Note that a cutoff wavelength that provides a single-mode condition in optical transmission is given by a wavelength set when R(xcex) defined by the method of measuring the cutoff wavelength (measuring it at a fiber length of 2 m) in ITU-TG. 650 becomes 0.1. Assume that light in a 1.55-xcexcm band as a signal wavelength range (light having a wavelength of 1,550 nm which is the central wavelength in this range, in particular) is to be transmitted. In this case, to satisfy the single-mode condition at the exit portion of the mode removing unit (R(1550 nm)=0.1), the mode removing unit must reduce the optical power of high-order modes to 1/40 or less of that of the fundamental mode at least at the exit portion of the unit. Note, however, that the dispersion-shifted optical fiber applied to the optical transmission line according to the present invention is much longer than a fiber length of 2 m that defines R(xcex), and hence a range of about 1/10 (R(1550 nm)xe2x89xa60.4) or less is allowed as a range in which transmission quality is not substantially affected, in consideration of the distance dependence of cutoff wavelength.
In order to reduce the optical power of high-order modes to 1/10 or less or 1/40 or less of the optical power of the fundamental mode, the mode removing unit preferably has a structure for winding a predetermined portion of a dispersion-shifted optical fiber with a diameter of 10 mm or more but 60 mm or less. Alternatively, the mode removing unit may have a structure for making a predetermined portion of a dispersion-shifted optical fiber meander with a radius of curvature of 5 mm or more. With this structure, high-order modes are greatly reduced, and the influences of high-order modes propagating through the dispersion-shifted optical fiber can be effectively reduced.
In order to effectively suppress a nonlinear phenomenon and implement wavelength division multiplexing transmission and soliton transmission as described above, a dispersion-shifted optical fiber that can be applied to the optical transmission line according to the present invention preferably has an effective area of 50 xcexcm2 or more with respect to light having a wavelength of 1,550 nm, a bending loss of 0.5 dB/turn or less at a diameter of 32 mm, and a cutoff wavelength of 2.0 xcexcm or more at a fiber length of 2 m. More preferably, the dispersion-shifted optical fiber according to the present invention has a dispersion slope of 0.04 ps/nm2/km or less with respect to light having a wavelength of 1,550 nm.
In order to obtain the above characteristics, in a dispersion-shifted optical fiber having a core region extending along a predetermined axis and a cladding region provided on the outer periphery of the core region, the core region may include a first core having a predetermined refractive index, a second core provided on the outer periphery of the first core and having a refractive index lower than that of the first core, and a third core provided on the outer periphery of the second core and having a refractive index higher than that of the second core. The cladding region may include an inner cladding provided on the outer periphery of the third core of the core region and having a refractive index lower than that of the third core, and an outer cladding provided on the outer periphery of the inner cladding and having a refractive index higher than that of the inner cladding. The dispersion-shifted optical fiber may include a core region having the above structure and a cladding region having the above structure.
As disclosed in Japanese Patent Laid Open No. 8-248251 (EP 0 724 171 A2), an effective area (Aeff) is given by
Aeff=2xcfx80(∫0xcex1E2rdr)2/(∫0xcex1E4rdr)xe2x80x83xe2x80x83(1)
where E is the electric field accompanying light propagating, and r is the distance from the core center in the radial direction.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples,while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.